The term “Client-Server” refers to a network application architecture which consists of client and servers communicating with each other via a communication protocol (e.g. HTTP).
Web refers an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URIs). The Web is made up of three standards: The Uniform Resource Locator (URL), which specifies how each page of information is given a unique “address” at which it can be found; Hyper Text Transfer Protocol (HTTP), which specifies how the client and server send the information to each other, and Hyper Text Markup Language (HTML), a method of encoding the information so it can be displayed on a variety of devices.
The Web works as follow: When a client's web-browser (e.g. MICROSOFT's INTERNET EXPLORER, APPLE's SAFARI) is opened, it communicates on behalf of the client (or computer being used) using Hypertext Transfer Protocol (HTTP) and makes a web page request. Once the request is sent, the client's web-browser computer will wait for a hypertext data stream from the server. When the server gets the request, it looks for the requested file and, if present, sends it to the client's web-browser as requested.
Servlets are programs running on a server which wait for requests coming from a client's web-browser and generate a response to this request (for example as a dynamically generated Web-page).